performing some operation using every member of the parameter pack is
as simple as expr(t...),
because t can be used directly in a pack expansion.
When the parameter pack appears as part of another type such as
std::tuple, the function parameter is not a
parameter pack so can't be expanded directly.
For example, in the function template

template<class... T>
void foo(std::tuple<T...> t);

performing an operation on every element of the tuple requires additional
code because t is not a parameter pack and
expr(t...) is not valid.
(By comparison, in Python unpacking tuples to pass to variadic functions
is built in to the language and is as convenient as func(*t)[Python].)

For unpacking a tuple in C++ the brute force alternative to
expr(t...) is to instantiate a class template with a
static member function, such as

DoExpr<0>::apply(t);

which calls

DoExpr<1>::apply(t, std::get<0>(t));

which calls

DoExpr<2>::apply(t, t0, std::get<1>(t));

and so on for each element until
DoExpr<sizeof...(T)>
is instantiated and can call expr(t0, t1, ..., tN).
This requires the user to write a recursive class template (and a
specialization to terminate the recursion) with a variadic member function
template and to use perfect forwarding (not shown here) to avoid
O(N2) copies.
To make a generic, reusable solution that can apply an arbitrary
function object to each element requires template partial
specialization and is probably beyond the abilities of many users of
std::tuple, so the alternative is to write non-generic code
and then rewrite it slightly differently when a different operation on
tuples is needed. This is a fairly complicated task and the end result is
not even a very natural way to write the desired operation.
The call to

DoExpr<0>::apply(t);

is far less expressive than something like expr(t...).

A simpler and more natural solution is to introduce a new parameter pack,
I, consisting of integers [0,N] and then use it in
a pack expansion such as expr(std::get<I>(t)...).
This requires no recursion and no class template specializations.
This proposal is to standardize a mechanism for creating a parameter
pack containing an integer sequence.

so that an instantiation like int_seq<0, 1, 2, 3>
can be passed to a function template that deduces a parameter
pack containing the integer sequence 0, 1, 2, 3.

In order to pass an int_seq to a function it must be
possible to construct the desired specialization either from the length of
sequence desired or from a parameter pack from which the length can be
deduced. This proposal offers both options, so that given the type
tuple<T...> the sequence [0, sizeof...(T))
needed to expand the tuple can be created by using the alias
make_int_seq<sizeof...(T)> (or equivalently
make_int_seq<tuple_size<tuple<T...>::value>)
or by using the alias to_int_seq<T...>.

With such a type in your toolbox it is easy to write the generic,
reusable solution described above for expanding tuple elements as
arguments to a function object:

This solution still uses recursion and template specialization, but
only in the implementation of make_int_seq, where users don't
need to write or understand it.
Standardizing the components makes it trivial for users to expand
tuple-like types in any situation, including the examples below.
A type like int_seq is a good candidate for standardization,
as similar types are already used in at least two major standard library
implementations and have been reinvented often.
Similar types feature in several answers to questions on sites such as
Stack Overflow
[Schaub][Wakely][Kühl],
where the number of questions about unpacking tuples
and the number of times types like int_seq are used in answers
demonstrates the demand for a solution to the problem.

Piecewise construction of std::pair requires two
tuples of arguments to be expanded to form constructor
arguments for the pair members. One implementation technique is to
delegate to a private constructor for pair that takes
two types like int_seq, for example

A note in [tuple.creation] says that implementations might support using
std::tuple_cat with other tuple-like type such as
std::array, but it's not required.
Users who need to implement it themselves can do so using a recursive
template, using tuple_cat to build up a tuple
element by element, but this creates lots of temporaries and instantiations.
An implementation is shown here for comparison with the
int_seq solution:

While I hope there will be broad agreement that some form of
integer sequence template should be standardized there are some
parts of the solution which might be harder to agree on.
The changes to the working paper proposed below are my preferred
solution but the specific details are less important than getting
some kind of template that make it easy to solve the problems
described above.

It's not obvious to me what type of integer should be used in the
parameter pack of the class template. The natural type for indexing into
tuples and arrays is std::size_t, but that type has a far
larger range of values than needed. I consider it unlikely that any
compiler will support parameter packs that can accept SIZE_MAX
arguments for the foreseeable future.
Another question is whether the integer type should be signed or unsigned.
Indices are naturally unsigned, but there could be other interesting uses
for integer sequences other than unpacking tuples and arrays which would
be more difficult if the values cannot be negative.
Conversely there might be valid use cases where the wrapping behaviour of
unsigned integers is preferable to worrying about overflow.

The proposed solution is to define a generic integer_seq
class template which can be instantiated as
integer_seq<int>
or
integer_seq<unsigned char>
or any other integer type.
By using alias templates to denote specializations for common types it is
just as convienent to use the generic template as it would be if it wasn't
parameterized by the integer type, for instance the examples above work
equally well whether int_seq<I...> is a specialization
of a class template or an alias for integer_type<int,
I...>.

There is no reason to restrict the values in an integer sequence to
consecutive integers starting from zero.
It is very simple to transform
int_seq<I...>
to int_seq<I+n...>
or int_seq<I*n>.
Such sequences would be useful to access only the even elements of an array,
for example, but such transformations are not necessary for the main
purpose of unpacking tuples. If the standard provides the
integer_seq type and simple metafunctions for generating
basic sequences users can more easily create custom sequences.

is sufficient, but it's simple to define type and
size members describing the type and to provide
append and next members for extending
sequences, as shown in the proposed changes below.

These members are not essential to the proposal and the suggestion
might be due to my own bias as the next member is used
by my implementation of the proposed changes below
[integer_seq]
i.e. in the unspecified details of how make_integer_seq
works.

The library provides a class template that can represent an integer sequence.
When used as an argument to a function template the integer sequence can
be deduced as a parameter pack and used in a pack expansion.

The alias template make_integer_seq denotes a
specialization of integer_seq to ease construction of an
integer sequence from the size of a template parameter pack.
The type make_integer_seq<T, N> denotes the type
integer_seq<T, 0, 1, ..., N-1>.